Wood finishers would like to use waterborne or powder coating finishing products to reduce their use of volatile organic compounds (VOCs). So far, however, powder coatings when applied to a natural wood substrate have resulted in uneven finishes suffering from bubbling or foaming in the coating caused by the outgassing of the wood, from a cloudy, blotched appearance and/or from orange peel or undesirable texture. Meanwhile, with waterborne wood coatings, a long, labor intensive process must be followed to get an acceptable finish.
To make a coating on wood from either waterborne or solvent borne coatings, several coats of sealers and topcoats are applied onto the stain to help to fill in the grain, while sealers are more sandable and may be sanded smooth before applying several topcoats to fill in the grain, each layer of which must be sanded. The liquid wood coating process is thus very labor intensive. In addition and unlike solvent borne stains, waterborne stains and coatings cause “grain swell” in natural wood, making the grain rise and requiring de-nibbing of the grain during sanding. Accordingly, waterborne coating of wood requires pre-sanding or grinding, application of stain, drying, application of sealer, drying, de-nibbing, sanding, and application of two or more topcoat layers, followed by drying, de-nibbing and sanding. Further, to improve the quality of the finish of any liquid wood coating, additional topcoats may be necessary.
Even with the labor intensive liquid wood coating process, coating buildup or “fatty pore” in the pores of large or open grain wood, such as oak, provides coatings appearing to have wavy rings around the pores. Such coatings are uneven and thus have an unnatural appearance.
A recent U.S. Pat. No. 6,296,939, to Kunze et al., discloses thermal and UV cured powder coating of heat sensitive substrates, such as wood, after they have been heated to from 80 to 120° C. with microwave radiation. This process eliminates much of the labor involved in wood coating. However, powder coatings formed directly on wood by this process foam or bubble. To solve this problem with coating wood substrates, Kunze et al. adds to the process applying a water based conductive paint to the substrate prior to microwave heating, followed by de-nibbing, sanding or grinding and powder coating the wood substrate. Nevertheless, the Kunze et al. coatings made with conductive paints and powder coatings cannot be less than 2.0 mils thick (50 μm) or they are undesirably uneven. None of Kunze's coatings avoid “fatty pore”. Further, regardless of thickness, the Kunze et al. coatings are disclosed only for wood “tiles” or flat pieces and give poor coverage of edges and of any substrate surface which is not flat, i.e. bevels, routered areas, trim and moldings. Still further, the conductive paint coatings of Kunze et al. have a cloudy appearance which makes them undesirable as clearcoats or tinted, translucent clearcoats.
Accordingly, there remains a need for clear or translucent powder coatings for natural wood which can provide an acceptable clear or tinted clear finish that is blemish or foam free. Further, there remains a need for a process for coating natural wood which reduces the labor required to make liquid stain on wood and provides a reliable clearcoat or tinted clearcoat finish on wood. In accordance with the present invention, the present inventors have surprisingly met these needs without any of the drawbacks of prior powder and liquid coatings for natural wood.